Hello, everyone. I have been coming here for all of my GNURadio and GRC problems, and everyone seems beyond knowledgeable. That being said, I have come to the end of my recourses and am forced to pose a question. I hope this hasn't already been answered, but I've been searching for a while, and have not found anything relevant to what I'm about to post.
I am trying to implement FSK over the air using the USRP1 boards within GRC. I have successfully sent large files between two computers, but am now trying to implement sending a .wav via the .wav source (and in the future, live audio with the audio source). In the case of pure bytes, the only thing that controls the sampling rate is the USRP, ie 128M(samples/second)/interpolation. However, in the case of a .wav file with a set sampling rate seems to be much harder. The following is the signal chain I'm using, and the chain of my logic as well. Transmit: >>Bit pipeline<< -Source: .wav at 44.1khz. -Multiply constant: 127 times the values within [-1,1] from the .wav source. -Float to short, -Short to float: This essentially truncates the float output [-127,127] of the .wav to 255 discrete values (since I don't know an easier way to do this within GRC) -Float to char: Now we take those discrete values from [-127,127] and map them to a byte each. >>FSK Modulation<< -Simple framer: Since the framer appends a barker code to the beginning of my packet that is 8 bytes, a byte for the sequence number, and another byte at the end of the packet (according to the code, the byte is 0x55, why this is there, I still don't know), I set the packet length to 4086 (ie 4086+10=4096 total packet length, a multiple of 128, as per http://old.nabble.com/RFX2400%2BUSRP-buffers-td19415046.html#a19415046 http://old.nabble.com/RFX2400%2BUSRP-buffers-td19415046.html#a19415046 ). SAMPLE RATE: I have assumed it to be about the same, since the bytes added are negligible compared to the packet length -Packed to Unpacked: Bits/chunk=1, therefore, for each byte put into the packed to unpacked, I receive 8 bytes out. SAMPLE RATE: 44.1kHz*8 = 352.8kHz. -Chunks to symbols: Since I'm only doing 2 level FSK, I only care about the value of the MSB, but I'm pretty sure the code for chunks to symbols reads all the bits anyway, so again, our sample rate is not affected. -Interpolating FIR filter: Set to length 8, with taps = 1. This spreads the effective mark and space bits out a little bit, ensuring full periods from the output of the FM mod block. Therefore, for each float in, I get 8 float out. SAMPLE RATE = 352.9kHz*8 = 2.8224MHz. -FM mod: This is where my confidence drops. I have selected my symbols as 2 and 4 in the previous block, and set sensitivity to pi/16. Therefore, for every "0" (symbol = 2), I get a full sinusoidal period in 8 samples, and 2 full periods in 8 samples of a "1" (symbol = 4). Now, I'm stuck. I have kind of assumed the sample rate at the output of the FM mod block to be equal to that going in, ie 2.8224MHz. The USRP broadcasts at 128M(sample/sec) / interpolation. Therefore, 129M/2.8224M ~ 45 (my interpolation). I guess this is where I am stuck. I'm not going to post the details of the receive side, as it's essentially exactly the same, but backwards, but I'm not getting a coherent .wav a the output of my speakers. It goes on for a small amount of time, and then the USRP outputs garbage (actually it looks like a sinusoid with very low frequency) for a split second, then back to the regularly scheduled program. I am wondering if there is, and how a buffer in the USRP (either block or actual hardware) treats a signal that is being "fed" either too fast, or too slow. And if so, do I have to add a rational resampler? I'm thinking I do, but I don't think anything will get the two sampling rates to match exactly I know this is a long one, but I'm at the end of the road of fully understanding FSK within GRC and the USRP board, and am pleased with my previous results. Now that there are two competing sampling rates, I don't know what to do. Thanks for your infinite wisdom, Justyn -- View this message in context: http://old.nabble.com/GRC-pure-FSK-with-USRP1-board-sampling-troubles-tp31650945p31650945.html Sent from the GnuRadio mailing list archive at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
